A communication scheme serving as a successor of W-CDMA and HSDPA, that is, a LTE system has been and is being discussed by a W-CDMA standardization organization 3GPP. In the LTE system, an OFDM (Orthogonal Frequency Division Multiplexing) scheme and a SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme are being investigated as downlink and uplink radio access schemes, respectively. See 3GPP TR 25.814 (V7.1.0), “Physical Layer Aspects for Evolved UTRA”, September 2006, for example.
In the OFDM scheme, a frequency band is segmented into multiple smaller frequency bands (subcarriers), and data is carried and transmitted over the individual frequency bands. According to the OFDM scheme, the subcarriers are densely arranged on the frequency band in such a manner that the subcarriers are partially overlapped with each other without mutual interference, resulting in fast transmission and highly efficient utilization of the frequency band.
In the SC-FDMA scheme, a frequency band is segmented, and different frequency bands are used among multiple terminals for transmission, resulting in reduced interference among the terminals. According to the SC-FDMA scheme, variations in transmit power are reduced, resulting in lower power consumption for the terminals and wider coverage.
Also, application of transmit diversity to the LTE system is being discussed. The transmit diversity may be advantageous to improved throughput and wider coverage for user equipment (UE) or a user apparatus having a high capacity and located at a cell boundary.
In the LTE system, however, the user equipment is not required to include two RF circuits. For the transmit diversity for uplinks, that is, from the user equipment to a base station apparatus, thus, some technique of implementing the transmit diversity by means of a single RF circuit is required.
For example, according to TSTD (Time Switched Transmit Diversity), two transmit antennas are switched between each other upon the lapse of a predefined time period for alternate uplink transmissions from the transmit antennas. The TSTD may be advantageous to channels to which no scheduling is applied, for example, to RACH (Random Access Channel).
In addition, a CL-ASTD (Closed Loop-based Antenna Switching Transmit Diversity) scheme for determining which antenna is to be used for transmission based on feedback is known. The CL-ASTS scheme is advantageous to channels where scheduling is applied. In the CL-ASTD scheme, a base station apparatus (eNB: eNodeB) measures a receive quality, such as CQI, of reference signals transmitted from antennas, selects an antenna to be used for transmissions based on the measured receive quality of reference signals, and feeds the selection to a user apparatus in an antenna selection command. See 3GPP R1-070097, “Performance Evaluation of Closed Loop-Based ANTENNA Switching Transmit Diversity in E-UTRA Uplinks”, January 2007 for example.